ICs are generally fabricated on one side of a semiconductor die. These dies are then used to power a diverse range of electronics products. IC packages conventionally include only one layer of ICs. Building multiple layers of ICs (or “stacking” the ICs) in the same semiconductor package can significantly reduce the lateral size of electronics and reduce the cost of manufacturing. As a result, stacked ICs are quickly gaining popularity for further extending the capabilities of electronics.
One feature used in building stacked ICs are through silicon vias (TSV). Through silicon vias are connections through the substrate of the die and may be used to connect a layer of ICs on one side of the substrate to an opposite side of the substrate where contacts may be provided for packaging the substrate. Through silicon vias occupy relatively small amounts of substrate area and do not otherwise interfere with circuitry built on the substrate thereby increasing the possible density of ICs. As the transistor density increases, the voltage supplied to the transistors decreases. These voltages are commonly smaller than the wall voltages available in most countries.
ICs are coupled to a voltage regulator that converts available wall voltages to the lower voltages used by the ICs. The voltage regulator ensures a predictable power supply is provided to the ICs. This is an important function because the tolerance of transistors to voltages under or over the target voltage is small. Only tenths of a volt lower may create erratic results in the ICs; only tenths of a volt higher may damage the ICs. As transistors in the ICs turn on and off, the power load changes rapidly placing additional demand on the voltage regulator. The distance between the voltage regulator and the ICs creates a long response time, preventing the voltage regulator from increasing power to the ICs instantaneously, especially when the transistors switch on and off millions or billions of times each second. Decoupling capacitors provide additional stability to the power supplied to ICs.
Decoupling capacitors attached in close proximity to ICs provide a local charge reservoir for the ICs. As demand on the power supply changes rapidly, the capacitor provides additional power and can refill at a later time when the power demand decreases. The decoupling capacitor allows ICs to operate at the high frequencies and computational speeds desired by consumers. However, as the transistor sizes have decreased and transistor densities increased, finding area on the ICs for decoupling capacitors has become difficult. Conventionally, capacitors are built from thick oxide transistors commonly used for I/O transistors. These capacitors are fabricated on the substrate to provide decoupling capacitance for the circuitry on the substrate. Thick oxide transistors offer very small values of capacitance in comparison to the large amounts of substrate area they consume that could otherwise be used for other circuitry.
Alternatively, through silicon vias may provide capacitance for decoupling. Through silicon vias include a conducting core and an insulating sleeve contained in a semiconductor substrate. If the conductor is connected to a supply voltage and the substrate is connected to a ground, then a capacitor is formed between the conducting core and the substrate. The capacitance is determined by the thickness of the insulator layer, the height of the through silicon vias, and the dielectric constant of the insulator layer. Generally, the dielectric constant is not easily changed. Therefore, the capacitance may be increased by decreasing the thickness of the insulating layer or increasing the height of the through silicon vias.
Semiconductor substrates experience charge depletion that creates an additional capacitance combined with the capacitance of the through silicon vias to form an effective capacitance. This effective capacitance will always be smaller than the smallest of the capacitance of the through silicon vias and the capacitance of the substrate. As a result, without changing the material of the substrate, only minor increases in effective capacitance may be gained from changing the design of through silicon vias.
Another type of via commonly found in ICs is a substrate or printed circuit board via. A substrate via is used to electrically couple several conductive layers in a printed circuit board substrate or packaging substrate. The vias are holes etched through the substrate that are plated with conductors and used to carry signals between layers. Multiple conductors may be plated in the vias separated by insulators to carry multiple signal lines through the via. However, these vias have low capacitance, if any. The insulator layer in the vias are thick (for example, 15-60 μm). The thickness prevents their use as decoupling capacitors. Additionally, depositing thin insulators with current techniques, namely lamination or build-up, is challenging.
Therefore, a new technique for providing decoupling of the circuitry from the die is needed that provides a higher degree of decoupling.